Magnetic recording and playing of a magnetic signal in a flexible magnetic disk comprising a sheet in a jacket, involving connecting a sheet-position determining part of a writing and/or reading apparatus to a central circular hole of the jacket and sheet, and rotating the sheet, is known. For the sheet to be stored in the jacket, a space is provided in the magnetic disk which is slightly larger than the sheet, and thus the sheet sometimes moves to an eccentric position in the jacket during use or storage, with the result that the central circular hole of the sheet deviates from the position of a position-determining part (collet) of the apparatus when the disk is loaded on the apparatus, and the sheet may be supported at an eccentric position at the position determining time.
FIG. 1A and FIG. 1B illustrate this type of flexible magnetic sheet, located in a rectangular jacket having circular open part 1a, with circular flexible magnetic sheet 2 having central circular hole 2a thereof in exposed the afore-mentioned central circular open part 1a, said sheet being stored in the jacket to allow free rotation.
Upon loading this flexible magnetic disk on the writing and/or reading apparatus, rotating part 3 (see FIGS. 2-4) of the position-determining part is fitted from under part of the sheet 2, and collet 4 is lowered from upper part of the sheet and is brought into the circular concave part 3a of rotating part 3 as shown in FIG. 2. At this time, the edge of central circular hole 2a does not always correspond to the position of circular concave part 3a of rotating part 3, and is many times out of proper alignment. If collet 4 is lowered in this state, some part of the portion surrounding the edge of circular hole 2a of the sheet 2 may be pinched between collet 4 and concave part 3a of rotating part 3, such that the sheet rotates eccentrically. As writing and reading cannot be carried out correctly unless magnetic disk sheet 2 rotates while being correctly centered corresponding to the central rotating axis, such eccentric pinching should very desirably be prevented. Moreover, there exists another problem, viz., that rattling occurs during rotation due to the absence of planarity of the sheet 2 when it is supported and rotated under such an eccentric pinched state.
Upon connecting the sheet 2 by lowering the collet 4, it is necessary that the portion surrounding the edge of the central circular hole of the sheet possesses an appropriate hardness, and that the friction coefficient (.mu.) between the sheet 2 and collet 4 as well as between the sheet 2 and rotating part 3 is small, in order that the sheet 2 is loaded correctly as shown in FIG. 4, not as shown in FIG. 3.
For this purpose, a method for reducing the friction coefficient by forming a protecting layer containing a polyolefin material polymerizable with ultraviolet light and an ester of an aliphatic acid on a surface portion surrounding the edge of the central hole of a flexible magnetic sheet where a sheet-position-determining part contacts the sheet is disclosed in U.S. Pat. No. 4,387,114. When lubricant of low molecular weight is employed as described in this method, however, the friction coefficient increases and the effect which prevents eccentric loading decreases under the atmosphere of relatively high temperature of from 40.degree. C. to 50.degree. C. and high humidity of from 70% to 80%. Furthermore, the lubricating effect decreases with repeated loading. The reason of this is probably that lubricant is wiped from the surface during the repeated use.